Current and expected trends in air travel have led to the design of a new generation of turboprop commuter aircraft which are expected to enter service in the mid 1980's. These are short haul aircraft, expected to service small airports located relatively close to populated areas. Accordingly, far-field noise restrictions for the aircraft will be quite demanding. The commuter aircraft will be used extensively by travelers in the initial and final legs of trips wherein the major portion of the distance traveled will be by modern, comfortable, wide-bodied turbofan aircraft. Accordingly, the demands on the commuter aircraft for safety, comfort, reliability and low cabin noise levels will be stringent.
To meet such stringent far-field and cabin noise restrictions, propeller tip speed must be kept to a minimum. However, since the new commuter aircraft are designed to operate from short runways, such low tip speeds must impart high blade thrust levels (lift coefficients) to the propellers at low blade weights (low blade solidities) for takeoff and climb modes of operation. Even with a minimization of tip speed, air speed over the propeller blade surfaces is necessarily quite high. To avoid pronounced shock waves and the attendant flow separation and sacrifice in performance resulting therefrom, it is necessary to maximize the critical Mach numbers associated with the aircraft propeller blade airfoil sections. For enhanced efficiency, high lift-to-drag coefficients at cruise conditions are also required.
In addition to satisfying the above-noted aerodynamic performance and noise requirements, the propeller blades must be capable of being manufactured with known production techniques and should exhibit a minimum risk of damage from both normal handling and impact with foreign objects.
State of the art airfoil families which define propellers and the like include the NACA Series 6 and Series 16 airfoils which heretofore have exhibited adequate aerodynamic and noise performance. However, for the new generation commuter aircraft noted hereinabove, the performance characteristics of propellers defined by such airfoil shapes are marginal at best. Newer airfoils such as the Lieback, Wortmann, Whitcomb supercritical, and GAW airfoils have been designed for special wing configurations and as such, are not suitable for general propeller use in that for the most part, these airfoils incorporate shapes undesirable for propeller manufacture from structural and fabrication standpoints.